Carbon fiber is broadly divided into pan-based and pitch-based fiber.
Currently, the PAN-based carbon fiber industrially manufactured by sintering acrylonitrile under the specific conditions is utilized as high-strength material (HP type). Since, however, the PAN-based fiber possesses low carbon content, a decomposition gas may be evolved and the yield is as low as 50 to 55%. Further, since the graphite structure in a high temperature is difficult to be developed, it is difficult to prepare carbon fiber with high modulus of elasticity though it is rather easy to prepare high strength products.
On the other hand, since the pitch-based carbon fiber is manufactured employing the pitch of coal and petroleum as raw material, the carbon content of spun fiber is as high as about 95% and the yield is also as high as 80 to 85%. Further, since the PAN-based carbon fiber is excellently characterized in its physical property by the occurrence of high modulus of elasticity, its development has been rapidly advanced.
Even for the pitch-based carbon fiber, when pitch is melted, spun and sintered as it is, the carbon fiber of optical isotropy can be obtained. The carbon fiber thus obtained is utilized as broadly employed carbon fiber (GP products) for reinforcing material of a structure because it is inexpensive and produces constant strength. The carbon fiber bearing optical anisotropy (mesophase) is to possess high modulus of elasticity (HM type) by spinning pitch having crystallizability because liquid crystals are arranged in the direction of a fiber axis in a shearing stress field during the spinning and huge graphite crystals can be produced by carbonizing the crystals.
Accordingly, product application being in conformity with these respective characteristics has been promoted; the carbon fiber simple substance is utilized as a filter, a catalyst, an electromagnetic shielder and the like; the carbon fiber in the composite material is utilized as reinforcing material of a matrix of a resin, a metal, carbon, ceramics and the like broadly in the field of the universe, aviation, leisure, sports, industry and the like.
The research has been advanced for employing the carbon fiber in combination with engineering plastics as electronic parts, automobile parts and structural material.
However, the tensile strength of the optically isotropic carbon fiber of these pitch-based carbon fiber is as low as 50 kg/mm to 100 kg/mm while the elongation rate thereof is as high as 2.5%. Although, on the other hand, the optically anisotropic carbon fiber has been obtained having the tensile strength of net less than 250 kg/mm and the modulus of elasticity of net less than 50 ton/mm, its elongation rate is about 0.5%.
However, in case that the carbon fiber is employed with a thermoplastic as composite material, a thermoplastic is ductile material and the reinforcing carbon fiber is small in ductility though it possesses large tensile strength and large modulus of elasticity so that the composite material exhibits the behavior of brittle material. Therefore, once a crack is generated, it is likely to invite final destruction to cause a large accident so that it is a severe problem how to elevate destruction tenacity for eliminating danger. The main of destruction of these plastics reinforced with the carbon fiber include destruction of a matrix, peeling of the fiber from the matrix, rupture of the fiber, pulled-out of the fiber and the like. Although actual destruction seems to occur by means of the combination thereof, among them the peeling between the fiber and the matrix and the pulled-out of the fiber are the main factors. Further, it is nearly impossible to employ the carbon fiber composite as an elastic body.
The reasons thereof may be that the carbon fiber is material of high linearity and that the surface of the carton fiber is so smooth that the bonding at the interface becomes a problem, and so on.
When the carbon fiber is employed as a simple substance, it is necessary to provide much more surface area and much more space in a constant volume of a filter, a catalyst and a like. Since the conventional carbon fiber is linear, it is molded with a binder for making a space after the fiber is woven as a net or piled like a mat. It is rather difficult to keep the space constant by employing the nets even if the woven ones are superposed. It is much more difficult to form a structural body provided with a constant cavity. The fiber is not at all employed in an application requiring a elastic structure.
Although the fiber fabric of the optically anisotropic carbon fiber can be formed as a radial structure, an onion structure, a random structure or a composite structure thereof by controlling the spinning conditions and the tensile strength, the modulus of elasticity, the elongation rate and the like can be changed by changing a hoat-treatment temperature, it is difficult to raise the elongation rate by more than 1% in all the instances.
Moreover, there arises a problem that the compression strength of the anisotropic carbon fiber is low. This is because the fabric of this fiber comprises broad carbon layers of which a face is aligned to parallel to fiber, and the strength of an a-axis and a b-axis of the carbon layer is high and that of a c-axis is low. Accordingly, in order to solve the problem, it is necessary to form a narrow carbon layer face as a PAN-based carbon fiber or to essentially change the texture of the optically anisotropic carbon fiber.